Generally for tires, especially a high level abrasion resistance and grip performance are demanded. As a method for improving abrasion resistance of tires, a method of blending to a rubber composition for a tire, a high molecular weight polymer (for example, one having a molecular weight of 250,000 or more, 500,000 or more, or 1,000,000 or more) and a carbon black as a filler is known. Also, it is generally known as a method for improving grip performance of tires to blend, to a rubber composition for a tire, a rubber having a high glass transition temperature (Tg) (for example, one having Tg of −20° C. or more), a carbon black having a large surface area (for example, one having a nitrogen absorption specific surface area of 110 m2/g or more), and the like.
However, in the case of using a high molecular weight polymer having a molecular weight of 250,000 or more for a rubber composition for a tire, there is a problem that a hardness of the rubber composition increases as a molecular weight increases, thereby deteriorating processability. Also, as a method of increasing a glass transition point of a rubber composition for a tire, there is a method of increasing a styrene content in a polymer. However, if a styrene content increases, for example, to 25% or more, a hardness of a rubber composition increases, which leads to deterioration of processability. Further, in the case of using a carbon black, if a carbon black having a large surface area is used, there are demerits such as lowering of its dispersion, and increase in a hardness of a rubber composition, resulting in deterioration of processability.
On the other hand, it is known that for improving processability, a process oil, a sticky resin, a liquid styrene-butadiene rubber or the like is added to a rubber composition. However, addition of them leads to decrease in strength of a rubber composition, and therefore, there is a limit in their amount to be used.
The present situation is such that so far there have not been obtained a rubber composition for a tire tread rubber enhancing both of abrasion resistance and grip performance to a high level and exhibiting excellent processability.
Myrcene is a natural organic compound and is a kind of olefin belonging to monoterpene. There are two kinds of isomers as myrcene such as α-myrcene (2-methyl-6-methyleneocta-1,7-diene) and β-myrcene (7-methyl-3-methyleneocta-1,6-diene), and when simply referring to myrcene, it usually indicates the latter one. In Patent Document 1, a polymer of myrcene is disclosed.
Farnesene is a kind of isoprenoid compound synthesized chemically by oligomerization of isoprene or dehydration reaction of nerolidol, and is used mainly as a perfume or its starting material (Patent Document 2).